Field of the Invention
The present invention relates generally to neutron detectors and, more specifically, to neutron sensing using a Gd-based scintillator coupled with two or more photodiodes.
Description of the Prior Art
Neutron sensing is an important task, which has implications across a wide range of modern societal needs, including global nuclear threat reduction. Neutron sensing is needed to assist in the identification and quantification of materials that possess neutron-emitting radioactive isotopes. More generally, there is also a need to quantify background neutron fluxes to accurately account for their effect on electronic devices, both transient and persistent effects alike. In today's integrated circuits (ICs) the density of devices has reached an all-time high, while the critical-charge upset-threshold continues to reduce. Thus, neutron-induced errors are an increasing issue for highly parallelized terrestrial computing applications as performed by data centers. Though limited in their interaction, neutrons, like most matter waves, are potentially harmful to humans making it important to quantify the flux of neutrons incident with humans working in environments that may lead to neutron exposure. In general, enhancements to the current state-of-the-art neutron sensing/detecting devices are highly sought after. Potential enhancements may result in performance improvements (sensitivity, dynamic range, energy resolution, gamma discrimination) or provide a rational means to overcome feasibility constraints (cost, power, size) and may have a wide or narrow range of applicability (e.g., background neutron sensing vs. neutron imaging of nuclear reactor cores).
A primary objective for neutron sensing is to enhance the efficiency and accuracy of identifying clandestine radioactive materials used in the production of nuclear weapons. Table 1 contains a list of common radioisotopes and the corresponding half-life, spontaneous fission rate, neutron emissions per fission event, and the number of neutrons per gram per second. The final column is the number of neutrons per cm2 per second incident on a detector approximately 11.5 ft (3.5 m) away from 1 kilogram of the material; this configuration approximates the conditions within a shipping vessel (20 ft×8 ft×8 ft), with the source located in the center and the detector in a corner. These data exemplify a primary challenge of neutron sensing/detecting for nuclear threat reduction, namely, low signal intensities. Consequently, physical approaches (selection of materials, size of detectors, etc.,) along with engineering strategies (signal processing, maximizing signal-to-noise) are paramount to achieving the desired level of sensitivity.
TABLE 1List of common radioisotopes used in the production ofnuclear power and/or nuclear weapons.Spont.Half-lifefissionNeutronsNeutrons/*Neutrons/Isotope(yr.)per decayper fissionkg-scm2-s235U7.04 × 1087.0 × 10−111.861.0 × 10−26.5 × 10−9238U4.47 × 1095.4 × 10−7 2.0713.68.8 × 10−7239Pu2.41 × 1044.4 × 10−122.1622.01.4 × 10−6240Pu65695.0 × 10−8 2.219.2 × 1056.0 × 10−2*The final column is the neutrons/cm2s incident from 1 kg of material held at 3.5 m assuming a point source geometry and 10% isotopic purity.
Neutrons interact with matter very differently than other forms of radiation such as x-rays, gamma-rays, electrons (beta particles), or ions (including alpha particles). As electric-charge neutral particles, neutrons are unaffected by the negative electric charge of electrons or the positive electric charge of atomic nuclei. This allows them to pass through most materials with virtually no interaction or deflection. The three dominant mechanisms by which neutrons interact with sensing material are elastic scattering, inelastic scattering, and capture reactions. Each of these events are characterized by their corresponding event cross-section, σ (also referred to as the differential cross section), which is the effective area within which a neutron, of a given energy, will be scattered or captured by the nucleus. Fissile neutrons (neutrons with high energy >1 MeV) interact with sensing material mainly through elastic scattering and to a lesser extent inelastic scattering, which can reach >10% of the elastic scattering cross section for heavy ions but is typically much less. To produce a measurable signal, scattering events must cause a displacement of the target nucleus; the interaction of this atom and the adjacent material is ultimately what generates the signal. Sensing fissile neutrons, therefore is challenging since elastic collisions result in a continuum of signal energies for neutrons above the threshold energy for atomic displacement, and no signal if the energy transferred by the neutron is below the threshold. In contrast, sensing of lower energy neutrons is made easier by the enhanced nuclear cross section associated with capture reactions. Moreover, capture reactions are inherently amplifying reactions as the resulting energy released is the sum of the incident neutron energy and the Q value of the reaction. Consequently, capture reactions yield reaction products that deposit a specific energy within the encompassing matter leading to characteristic signal features that are conducive to sensing.
Neutron sensing is fundamentally limited by the neutron cross-section of the neutron-sensing medium of the detector. Maximizing the neutron cross-section, mass density, and total volume are effective means to maximize detector efficiency. In particular, isotopes with large neutron cross sections are included in Table 2; thermal neutrons are neutrons with energies corresponding with ambient thermal energy (<0.026 eV).
TABLE 2Thermal neutron cross section of commonly usedneutron sensing isotopes.Thermal NeutronIsotopeCross Section (b)3He5,3306Li94010B3,840157Gd255,000235U585
A typical neutron event proceeds as 01n+X→Y+b, where Y is the heavier ion which typically remains in the original lattice location, and b is of lower mass and emitted with an energy that corresponds with balance of the excess energy of the reaction. Similar to elastic scattering events, the electronic excitations caused by the emitted high-energy charged particle are what lead to a measurable response. The precise energy of the emitted particles of capture reactions, however, is much more conducive to pulse-shape discrimination techniques, leading to potentially higher efficiency and gamma discrimination.
The form in which energy is released by a neutron capture reaction has a large role in defining the subsequent detection mechanism used to generate a signal response. For instance, 157-Gd and 10-B neutron capture reactions proceed as follows:
                             0        1            ⁢      n        +                                                                     ⁢          64                157            ⁢      Gd        ->      {                                                                                                                                                                                                             -                          1                                                                                                                                                          ⁢                          0                                                                    ⁢                      e                                        ⁡                                          (                                              0.079                        ⁢                                                                                                  ⁢                        MeV                                            )                                                        +                  γ                                                                                                                                                                                                                                                                         ⁢                        62                                            154                                        ⁢                    Sm                                    +                  α                                                                                                                                                                                                                                                                         ⁢                        63                                            157                                        ⁢                    Eu                                    +                                                                                   1                      1                                        ⁢                    H                                                                                ⁢                                          ⁢                                                   0              1                        ⁢            n                          +                                                                                                     ⁢              5                        10                    ⁢          B                    ->              {                                                                                                                        Li                                            *                                              3                        7                                        ⁡                                          (                                              1.472                        ⁢                                                                                                  ⁢                        MeV                                            )                                                        +                                      α                    ⁡                                          (                                              0.840                        ⁢                                                                                                  ⁢                        MeV                                            )                                                                                                                                                                                                                                       3                        7                                            ⁢                      Li                                        ⁡                                          (                                              1.776                        ⁢                                                                                                  ⁢                        MeV                                            )                                                        +                                      α                    ⁡                                          (                                              1.013                        ⁢                                                                                                  ⁢                        MeV                                            )                                                                                                                                                                                                             4                      9                                        ⁢                    B                                    +                                                                                   1                      2                                        ⁢                    H                                                                                                                                                                                                                                                                                           ⁢                        4                                            10                                        ⁢                    Be                                    +                                                                                   1                      1                                        ⁢                    H                                                                                .                    
In both instances, the first reaction has the highest probability (>97% for neutrons with energies of <10 MeV) and is the only reaction considered here. The 79 keV beta-particle emitted in the 157-Gd reaction has a comparably smaller linear energy transfer rate (LET) in matter than heavier charged particles like the alpha particle and Li-ion emitted in the 10-B reaction. Furthermore, the long range of gamma-rays makes it possible for a fraction of the reaction energy to leave the detector entirely. This reduces the magnitude of the response and would broaden the energy response signal (a histogram counting the number of events within a given range signal amplitude [energy]). This example illustrates that maximizing the neutron cross section of the sensing material is only part of the neutron detector design challenge. Sensing the energy released by the reaction products (or the recoil atoms in the case of elastic collisions) is equally as critical and requires a particle specific device tailoring for maximum efficiency. In practice, many solid-state neutron detector approaches, sacrifice the larger neutron cross-section of 157-Gd for the more easily sensed reaction products of isotopes like 10-B and 6-Li, among others.
Directly sensing the effects of the reaction products (direct sensing of reaction products is effectively 2-step neutron sensing) as they transport through matter may be accomplished by any means that can separate and measure (count) the resulting excited electrons. In regards to solid-state devices, this is often achieved by using a semiconductor diode comprised of a p-type semiconductor adjacent to an n-type semiconductor. In these devices, the internal field of the diode separates carriers and outputs a current that is proportional to the total energy lost by the reaction products of the nuclear event within the active region. The active region being the region of the device in which energy deposited (of sufficient magnitude) has a measurable effect. All other regions are considered dead space. Charge coupled devices (CCDs) are effective at counting charges that becomes stored in a floating dielectric material—the active region. Arrays of these structures may be used to increase the sensing area, or under large neutron fluxes, be used to generate neutron images. Recently, static random access memory (SRAM) devices have been used to count the number of reaction products with sufficient energy to cause a bit-flip to occur. Once flipped, the neutron event becomes stored indefinitely in the memory allowing periodic sampling to take place on the detector at a later time. Dosimetry type approaches using metal-oxide-semiconductor (MOS) based transistors rely on the reaction products to create a population of trapped charges in the gate dielectric layer. The trapped charges shift the MOS transistor transfer characteristics leading to a measurable response that, once calibrated, provides a measure of the total number of incident neutrons received.
In general, direct-conversion solid-state neutron sensors and dosimeters rely on reaction products that transit through and deposit a critical amount of energy within the active region of the device to register a neutron absorption event. Therefore, close proximity of the neutron sensing material with the active region is important for maximizing the neutron signal. The maximum range of typical reaction products, e.g., alpha or beta particles, is less than 100 μm (5 MeV alphas have a range of 24.2 μm in Si). Therefore, an optimal thickness of the sensing material balances the need to increase the thickness to maximize neutron absorption with the requirement that the reaction product must reach the active region. For thicknesses of this scale, incomplete absorption of the incident neutron flux may occur, which reduces the intrinsic efficiency of the device.
Many novel device structures have been proposed to circumvent the geometric losses. Regarding diode-based detectors, the planar geometry device design is altered through various means to form two-dimensional and three-dimensional structures that aim to simultaneously maximize sensing-material volume while minimizing the average distance from the sensing material to the active region. Other approaches incorporate large-neutron cross section elements within the solid-state converter, which has the potential to increase the intrinsic neutron detection efficiency to unity. In general, these approaches are hindered by lower quality materials and subsequently lower efficiency devices. The goal of these neutron detectors is to mitigate the geometric losses while maintaining the favorable form factor, low-cost (comparably), and low-power attributes of devices based on solid-state converters.
Scintillators, crystals that luminesce under exposure to ionizing radiation including alpha, beta, x-ray, gammas, etc., provide an alternative strategy for neutron detection. Scintillator crystals have been extensively used in nuclear science because of their ability to absorb ionizing radiation and rapidly convert it to light which is conducive to measurement via photodiodes, photomultipliers, microchannel plates, etc. Scintillators act as particle amplifiers by converting the energy emitted from a single ionizing particle into 100s to 1000s of photons. Highly efficient crystals can exceed 100,000 photons per 1 MeV of incident ionizing radiation. Therefore, a neutron capture reaction with Q=5 MeV can have an effective neutron-to-photon quantum yield (ηn→hv) in excess of 100,000, depending on the LET and range of the reaction product. With a low noise, high-speed detector, the scintillator emission output signal can be calibrated to provide the precise energy of the sensed particle, and using pulse-shape discrimination techniques, identify the specific nature of the particle (i.e., neutron, gamma, x-ray, beta, alpha, etc.). Integrating isotopes with large neutron cross sections, such as 6-Li, 10-B, and 157-Gd is important to sensitizing scintillators to neutron fluxes. A recent work reports exceptional light output, rise time, and decay time for Lutetium gadolinium halides doped with Ce. Typical light output, rise times, and decays times were 80,000 photons/MeV, <1 ns, and <50 ns, respectively making the material applicable to neutron sensing and pulse-shape discrimination techniques. (Glodo et al., “Mixed Lutetium Iodide Compounds,” Nuclear Science, IEEE Transactions on, 55, 3 1496-1500 (2008) and U.S. Pat. No. 7,755,054 (2008)).
Typical detection methods utilize fast photomultiplier tubes (PMTs), photodiodes, or microchannel plates to sense the photons emitted from a scintillator crystal. These devices are subsequently connected to additional electrical analyzers, such as a multichannel analyzer (MCA) for pulse-shape analysis, or directly to an instrument for voltage/current read-out. Advances in digital electronics have enabled a significant reduction in the size and power necessary to process the scintillator output. Handheld, battery operated MCAs are commercially available. However, PMT based sensing approaches remain unfavorable because of the high voltages and size of the tube. Conversely, solid-state based sensing devices are highly scalable, making them amenable to standalone, battery powered operation. However, solid-state devices based on Si photodiodes (PIN diodes) are inherently limited in speed and leakage current due to the long lifetime and relatively narrow bandgap of Si. Silicon avalanche photodiodes improve the lifetime and leakage current but at the price of much larger biases ranging from 10-100 V. Furthermore, devices based on Si have a very strong temperature dependence which increases the dark current by an order of magnitude for every 10° C. The key to achieving a low-power, low-noise, temperature independent nuclear detector is to utilize high efficiency photodiodes based on wide bandgap semiconductors that are operated in (unbiased) photovoltaic mode.